The present invention relates to apparatus and methods for detecting wet or icy conditions due to precipitation accumulating on a surface of a pathway, such as a highway, a bridge, an airport runway, or a building floor. More particularly, the present invention relates to precipitation sensing apparatus which can be embedded in a pathway for sensing the presence, type or amount of atmospheric precipitation and methods of its manufacture and operation.
Apparatus which detects wet and icy surface conditions is a potentially indispensable aid to motorists, highway departments, airport managers, pilots and others. A motorist can be made aware of an icy condition on a bridge deck before crossing it and take the necessary precautions. Likewise, once an airport manager is aware of a slippery or icy condition on one or more of the airport runways, planes can be diverted to a non-icy runway and deicing procedures initiated on the affected runways. Motorists can be warned of a light accumulation of water on a dirty highway, which is a hazard almost as well known as it is difficult to see.
When an aircraft lands on a runway covered with water, a condition called hydroplaning can cause a dangerous skid. The minimum depth of water at which hydroplaning first occurs is inversely related to the landing speed of the aircraft, generally speaking. Airport management faces a safety dilemma since aircraft cannot be allowed to land on runways which are unsafe for them. However, unnecessary closing of runways must be avoided to prevent congestion of the available airspace and consequent reduction of safety in the air. Consequently, only those runways should be closed which are actually hazardous to the particular aircraft requesting clearance to land. Accurate precipitation depth information over a wide range of depths is essential to safety-related decisions, and warning information is also needed as soon as there is incipient icing of a wet pathway.
In coassigned U.S. Pat. No. 4,135,151, precipitation on the sensor is identified as water, slush, or ice by a capacitance and conductance approach. In coassigned U.S. Pat. No. 4,281,286 at least two different frequencies are used in a capacitance and conductance approach, depending on whether impurities are or are not present in the precipitation. Both of these patents U.S. Pat. Nos. 4,135,151 and 4,281,286 are specifically incorporated herein by reference.
French patent No. 2078982 suggests an assembly with a capacitance and resistance approach. The resistance sensor has concentric electrodes. When deicing agents are present on the ground, these modify the conductivity of the surface water and in order to compensate this effect the resistance between two other concentric electrodes comparable in dimensions to the electrodes of the resistance sensor is used as a reference to correct a depth signal.
U.S. Pat. No. 3,613,063 shows two concentric metal electrodes separated by an air gap for entry of precipitation. The outer metal electrode has a dished, concave shape in one version.
U.S. Pat. No. 4,335,613 shows an elongated ultrasonic sensor placed in a groove in a road surface.
Gaps and grooves present self-evident problems of dirt retention and the need for frequent maintenance. Moreover, devices with gaps or grooves are susceptible to damage by repeated freezing and thawing cycles in harsh environments and also by tire impact and wear. Compensation for impurities by multiple electrodes of comparable dimensions results in a restricted range of depth measurement.
Also, conductance-based sensors continue to be subject to mingled effects of both precipitation depth and impurities on measured conductivity. This mingling of effects presents a problem of determining and isolating the contributions made by precipitation depth and impurity concentrations respectively. Moreover, impurity concentration can vary markedly over even relatively short distances, casting a cloud of uncertainty on the prior multiple electrode approaches. Chemical reactions on sensor electrodes exposed to the elements causes dielectric deposits that further complicate the measurement problem. Evaporation of a wet surface may result in a very thin precipitation layer that has very low conductance but is still detectable by a capacitance sensor, complicating the determination of precipitation type.
The ultrasonic measurement approach of coassigned U.S. Pat. Nos. 4,769,160 and 4,750,117 offers an attractive alternative technology in depth sensing. However, it is desired to make further improvements in conductance sensors so that these also are utilized to their full potential for accurate characterization of precipitation, for detecting impurities and depth sensing as well.